t ten thousand pounds to
the square inch. As the weight of the iron is known, and its tensile
strength is estimated at sixty thousand pounds per square inch, this
estimate, which is technically called "a factor of safety" of six, is a
very safe one. In other words, the bridge is planned and so constructed
that in supporting its own weight, together with any load of locomotives
or cars which can be placed upon it, it shall not be subjected to a
strain over one-sixth of its estimated strength.

[Illustration: NEW RIVER BRIDGE ON ITS STAGING.]

After the plan is made, working drawings are prepared and the process of
manufacture commences. The eye-bars, when made, are tested in a
testing-machine at double the strain which by any possibility they can
be put to in the bridge itself. The elasticity of the iron is such that
after being submitted to a tension of about thirty thousand pounds to
the square inch it will return to its original dimensions; while it is
so tough that the bars, as large as two inches in diameter, can be bent
double, when cold, without showing any signs of fracture. Having stood
these tests, the parts of the bridge are considered fit to be used.

[Illustration: BRIDGE AT ALBANY.]

When completed the parts are put together--or "assembled," as the
technical phrase is--in order to see that they are right in length, etc.
Then they are marked with letters or numbers, according to the working
plan, and shipped to the spot where the bridge is to be permanently
erected. Before the erection can be begun, however, a staging or
scaffolding of wood, strong enough to support the iron structure until
it is finished, has to be raised on the spot. When the bridge is a large
one this staging is of necessity an important and costly structure. An
illustration on another page shows the staging erected for the support
of the New River bridge in West Virginia, on the line of the Chesapeake
and Ohio Railway, near a romantic spot known as Hawksnest. About two
hundred yards below this bridge is a waterfall, and while the staging
was still in use for its construction, the river, which is very
treacherous, suddenly rose about twenty feet in a few hours, and became
a roaring torrent.

[Illustration: LA SALLE BRIDGE.]

The method of making all the parts of a bridge to fit exactly, and
securing the ties by pins, is peculiarly American. The plan still
followed in Europe is that of using rivets, which makes the erection of
a bri